The invention relates to chemical mechanical planarizing (CMP) formulations for removing barrier metals and, more particularly, to polishing compositions for selectively removing barrier metals in the presence of interconnect structures in integrated circuit devices.
In recent years, the semiconductor industry has increasingly relied upon copper electrical interconnects in forming integrated circuits. These copper interconnects have a low electrical resistivity and a high resistance to electromigration. Since copper is very soluble in many dielectric materials, such as silicon dioxide and low-K or doped versions of silicon dioxide, a diffusion barrier layer is necessary to prevent the diffusion of copper into the underlying dielectric material. Typical barrier materials include, tantalum, tantalum nitride, tantalum-silicon nitrides, titanium, titanium nitrides, titanium-silicon nitrides, titanium-titanium nitrides, titanium-tungsten, tungsten, tungsten nitrides and tungsten-silicon nitrides.
In response to increasing demands for high density integrated circuits, semiconductor producers now fabricate integrated circuits containing multiple overlying layers of metal interconnect structures. During device fabrication, planarizing each interconnect layer improves packing density, process uniformity, product quality and most importantly, enables manufacturing of multiple layer integrated circuits. Semiconductor producers rely upon chemical-mechanical-planarizing (CMP) as a cost effective means of producing flat substrate surfaces. The CMP process is typically carried out in a two-step sequence. First, the polishing process uses a “first-step” slurry specifically designed to rapidly remove copper. For example, Carpio et al., in “Initial Study on Copper CMP Slurry Chemistries” Thin Solid Films, 262 (1995), disclose the use on a 5 weight percent nitric acid solution for efficient copper removal. Similarly, Kondo et al., in U.S. Pat. No. 6,117,775, disclose the use of nitric acid and BTA for copper removal.
After the initial copper removal, a “second-step” slurry removes the barrier material. Typically, second-step slurries require excellent selectivity to remove the barrier material without adversely impacting the physical structure or electrical properties of the interconnect structure.
Because it was traditionally perceived that alkaline polishing slurries have a much higher tantalum and tantalum nitride removal rate than acidic slurries, commercial second-step slurries typically have a neutral-to-basic pH. Another factor pointing to the advantages of neutral to basic pH barrier metal polishing slurries relates to the need to preserve the metal overlying the barrier metal during the second-step polishing. The metal removal rate should be very low to reduce dishing of the metal interconnects.
In acidic slurries that include oxidizing agents, copper tends to have both a high removal rate and a high static etch rate. Cote et al. however, in U.S. Pat. No. 6,375,693, disclose an acidic CMP slurry for barrier materials. The slurry of Cote et al. operates with a hydrogen peroxide oxidizer, a benzotriazole inhibitor and a sulfated fatty acid at a pH range of 2 to 7.5. Similarly, Wojtczak et al., in U.S. Pat. No. 6,409,781, disclose an acidic polishing slurry that relies upon a potassium iodate oxidizer, iminodiacetic acid as the copper corrosion inhibitor and nitric acid as the copper activator to selectively polish the barrier material. Unfortunately, acidic polishing slurries can have stability problems wherein the slurry undergoes a yellowing or other color change. This yellowing occurs after the addition of hydrogen peroxide to the acidic polishing slurry. Furthermore, since polishing slurries typically have a pot-life of several days, this yellowing phenomena can result in a slurry that darkens with time. And this color change can be an indicator of changing polishing performance that can result in decreased semiconductor wafer yields.
Future low k and ultra-low k integrations of the IC structure will require low metal and dielectric losses in the CMP step. Accordingly, a selective slurry for barrier removal will most probably be adopted. While neutral-to-basic polishing slurries have advantages known to those skilled in the art, such as those described above, these slurries also tend to have low tantalum removal rates. In addition, because tantalum is readily oxidized, the oxidizing agents in the slurry can react with the tantalum to form an oxide layer on the surface. In view of the above, there exists a need to provide a second-step slurry that possesses a high removal rate of barrier materials, excellent selectivity to interconnect metals and controlled removal of dielectric materials. In addition, there is a need for a slurry with sufficient stability over time to avoid the adverse color change associated with acidic barrier slurries.